The backstop – part of the withdrawal agreement negotiated by former prime minister Theresa May which has been rejected by Parliament three times – is an insurance policy to prevent a hard border in Northern Ireland.

If implemented, it would see Northern Ireland stay aligned to some rules of the EU single market.

In his letter, Mr Johnson described the arrangement as “inconsistent with the sovereignty of the UK” and insisted it could not form part of a withdrawal agreement.

He also warned that it risked “weakening the delicate balance” of the Good Friday peace agreement.